Direct and Indirect Fitness: The Foundation of Kin Selection

Kin selection is based on the concept of inclusive fitness, which combines:

• Direct fitness: The reproductive success an individual achieves through its own offspring.

• Indirect fitness: The additional reproductive success gained by helping relatives reproduce, thereby passing on shared genes.

An altruistic gene can spread in a population if it increases the actor’s genetic representation in the next generation through either or both of these pathways.

Hamilton’s Rule and the Spread of Altruism

W.D. Hamilton formalized this idea mathematically with Hamilton’s rule:

$$rb>c$$

where:

• $r$ = coefficient of relatedness between actor and recipient

• $b$ = benefit to the recipient

• $c$ = cost to the actor

This rule predicts that a gene for altruism will be favored when the benefit to the recipient, weighted by relatedness, exceeds the cost to the actor.

Which Statement Is Correct?

Let’s evaluate the options:

• Option 1: Only direct fitness is considered—this ignores the crucial role of indirect fitness in kin selection.

• Option 2: Only indirect fitness is considered—this also misses the full scope, as direct fitness is part of inclusive fitness.

• Option 3: Both direct and indirect fitness are included—this matches the definition of inclusive fitness and the predictions of kin selection.

• Option 4: States that altruism cannot spread—this is directly contradicted by the theory and evidence supporting kin selection.

Conclusion

A gene for altruism will spread in the population if the act of altruism increases the actor’s gene in the next gene pool through direct or indirect fitness. This is the essence of kin selection and inclusive fitness theory, explaining the evolutionary success of altruistic and cooperative behaviors in nature.

Correct answer: (3) A gene for altruism will spread in the population if the act of altruism increases the actor’s gene in the next gene pool through direct or indirect fitness.